The independent variable is the factor that an experimentor will manipulate to achieve different results. In this case, the experimentor wants to understand the effects of air temperature on the rate of photosythesis. So, in order to be sure that the results are related to air temperature, the experimentor must manipulate only this factor, maintaining the other variables (such as amount of carbon dioxide, volume of oxygen produced and the number of corns) the same. Therefore, the correct answer is (1) air temperature at which the corn plants were grown.